1. Field of The Invention
In general, this invention relates to electrical logging of formations surrounding a borehole; more particularly, it relates to measuring formation resistivity by processing signals induced in receiving antennas by electromagnetic waves that are caused to propagate through the formation.
2. The Prior Art
Data concerning how an electrical parameter such as formation resistivity varies with well depth provide useful clues in exploring for oil and gas bearing beds. Resistivity may be expressed as the ratio of voltage gradient (e.g., voltage difference per meter), to current density (e.g., amperes per square meter), and is generally expressed in units of ohm-meters.
Over the years, many formation-resistivity measuring systems have been developed. The known systems may be classified in numerous ways. According to one basis for classification, a system can be categorized as either a wireline system or as a measure while drilling (MWD) system. Such systems can also be categorized with respect to frequency of operation of electrical signals used in effecting measurement of formation parameters.
Measuring while drilling has significant, long-recognized advantages. For the measurement of formation resistivity, a MWD system is particularly advantageous in reducing or eliminating the adverse effect that can be caused if drilling fluid (referred to as "mud") has had sufficient time to invade the formation before the resistivity measurement is made. A MWD system is advantageously incorporated in a logging collar section or sub that is positioned in the drill string very close to the drill bit. Thus, relatively little time elapses from when the drill bit cuts through a region and when the sub is brought into position to effect a measurement of a parameter of the formation in that region. Another advantage of a MWD system is that data for any formation parameter it measures that can be communicated to the surface during drilling operations via mud pulse telemetry techniques.
Such advantages, however, can be gained only if the MWD system is capable of withstanding the extremely adverse environmental conditions prevailing down hole while drilling. The adverse environmental conditions involve high temperature and shock. Further, during the drilling operation, mud is circulated under high pressure to flow down through a passageway within the drill string to, and out of the drill bit, and then to flow back up in the annular space between the drill string and the wall of the borehole, carrying cuttings to the surface. Various elements of a MWD system must be contained in an electronics housing, which is a sealed pressure vessel or barrel, that is anchored in place within the MWD sub and that protects components such as electronic circuits from exposure to the high pressure mud. Further, any element of the MWD system that is exposed to the upwardly flowing mud, and the whipping action of the drill string against the inner wall of the borehole, must be extremely abrasion resistant.
Another difficulty associated with a MWD system is that the electrical power for the MWD system is generated within a downhole drilling segment by a turbine-driven generator, the turbine being driven by the downflowing mud. Because the electrical power is generated inside the drill string segment while high pressure drilling mud is circulating down inside the drill string segment and up about its outside, complexities arise in distributing electrical power and electrical signals to various components of the MWD system.
In contrast to a MWD system, far fewer problems need to be addressed in a well logging system, commonly called a wireline system, that is used while drilling operations are suspended. Because the mud is stationary while drilling operations are suspended, various elements of a wireline system are not subjected to the adverse conditions discussed above. One minor exception is that downhole temperature is somewhat higher while the mud is stationary than while the mud is circulating and to some extent providing cooling. The environmental conditions of use of a wireline system, in addition to being generally more benign, enable substantially more control over distribution of electrical power. In a wireline system, a generator is located at the surface, and the electric power it generates is easily supplied to downhole electronics.
As stated above, another way of classifying a system is on the basis of the frequency of operation of an electrical signal used to effect the measurement. With respect to the lowest end of the frequency spectrum, there are electrode systems that involve a frequency of operation in the neighborhood of about 1 KHz. Each of these low frequency systems relies upon conduction of current through the mud as part of a current flow path that also includes electrodes and the surrounding formation. Because the mud forms part of this current flow path, an electrically conductive mud such as a water base mud is necessary for proper operation of any of these systems. In many drilling operations, it is undesirable to use such a water base mud, and instead it is desirable to use an oil base mud that has very high resistivity.
Examples of wireline electrode systems are described in two papers authored by Hubert Guyod. One of these, titled "The Shielded Electrode Method," appears in the December, 1951 issue of World Oil, at pages 111-116. The other, titled "Factors Affecting the Responses of Laterolog-Type Logging Systems (LL3 and LL7)," appears in the February, 1964 issue of the Journal of Petroleum Technology, at pages 211-219. Examples of MWD electrode systems are described in U.S. Pat. No. 4,570,123 to Grosso. An improved MWD electrode system has been developed by the assignee of this invention and is disclosed and claimed in a U.S. patent application Ser. No. 025,937, filed Mar. 16, 1987, entitled "WELL LOGGING SYSTEM EMPLOYING FOCUSED CURRENT IN MEASURING RESISTIVITY WHILE DRILLING"; the inventors being J. Meisner, et al.
With respect to the next frequency range in the spectrum, there are induction systems that involve a frequency of operation in the neighborhood of 20 KHz. An induction logging system generates a magnetic field in the formation to produce a secondary current flow in the formation. The secondary current flow sets up a second magnetic field which induces current in receiving coils in proportion to the secondary current flow in the formation and thus the induced current is directly proportional to the resistivity of the surrounding formation.
An induction logging system uses large diameter coils to obtain the necessary coupling. To apply induction logging techniques in a MWD system, inductive logging coils must be mounted in or about a drill collar in a drill string and that portion of the collar must be non-conductive. It is difficult to build a non-conductive collar that has the structural integrity and strength necessary for use in a drill string.
With respect to a much higher range of frequencies in the spectrum, there are electromagnetic wave propagation (EWP) systems that involve a frequency of operation in the range of about 500 KHz to about 4 MHz. An EWP system is disclosed in U.S. Pat. No. 3,551,797 to Gouilloud et al. The EWP system disclosed in the Gouilloud patent is a wireline system having a transmitter and receivers for measuring formation parameters, and utilizing phase comparison and amplitude. U.S. Pat. Nos. 4,107,597 and 4,185,238 also show EWP wireline systems.
Each of the foregoing wireline systems involves a non-conductive sonde. Because the sonde is non-conductive, it does not significantly reduce the signal strength of the electromagnetic wave signal used to effect the measurement of formation resistivity. Circumstances are different in the case of a MWD system in which electromagnetic wave propagation is used to accomplish the measurement of formation resistivity. A MWD system necessarily involves a metal drill collar, and because the metal drill collar is highly conductive it can significantly reduce the signal strength of the receiver signals derived from the electromagnetic wave signal used to effect the measurement. The problem of dealing with very low signal strengths can be aggravated by the presence of electrical noise.
A published U.K. Patent Application No. GB 2,146,126A is directed a specific electrostatic shielding arrangement for reducing the adverse effect of noise in a MWD system that uses electromagnetic wave propagation for measuring formation resistivity. A paper relating to this MWD system was presented at a SPE conference in San Francisco, Calif., Oct. 5-8, 1983; this paper is titled "The Electromagnetic Wave Resistivity MWD Tool"; its authors are P. F. Rodney et al. According to the published U. K. patent application, the preferred embodiment of the specific electrostatic shielding system is provided in a MWD EWP system that includes a drill collar that has an outside diameter of 17.8 cm, and has two axially-spaced apart cylindrical annular recesses each of which has a diameter of 14.6 cm. Each of the recesses is filled in with nitrile rubber. A transmitting antenna is embedded in the nitrile rubber that fills in one of the recesses, and two axially-spaced apart receiver antennas are embedded in the nitrile rubber that fills in the other recess. Each antenna has an inner diameter of 15.75 cm. Thus, there is a minimum spacing of about 0.55 cm. between an antenna and the drill collar. The axial spacing between the transmitting antenna and the nearer of the two receiving antennas is 24 inches, and the more remote antenna coil is an additional 6 inches further away. Each antenna is almost completely surrounded by a corresponding one of three electrostatic shields that are electrically isolated from the metal drill collar and electrically connected by conductors in coax cables extending to signal processing circuitry to ground (0 volts).
The performance capabilities of a well logging system can be judged in terms of various factors. One factor is ease of use; others are depth of investigation, dynamic range, resolution with respect to delineating narrow beds, and the extent to which measurements are independent of extraneous matters such as borehole effects.
As stated above, some well logging systems are not adapted for use in circumstances in which an oil base mud is being used. As to prior art EWP systems that are adapted for use in such circumstances, there are various problems. One of these problems is that the response of each antenna has to be extremely stable with respect to temperature variations. The known design principles applicable to such a prior art system are to effect a tradeoff of antenna sensitivity in favor of antenna stability. This in turn results in other sacrifices. In this regard, it is desirable to provide substantial axial spacings between the transmitting antenna and the receiving antennas. However, the relatively low sensitivity of the antennas used in prior art systems imposes a limit on how far apart the antennas can be spaced.
Such shortcomings are widely recognized, and there has been a longstanding need for an improved EWP system to overcome these shortcomings.